The present invention relates to halogen lamps having a sealed light emitting chamber and a tungsten filament positioned within the chamber. More specifically, the present invention relates to a pelletized lamp fill material for delivering precise amounts of rhenium and a halogen into halogen lamps and methods of dosing halogen lamps with lamp fill material.
Halogen lamps have found widespread use in a variety of applications such as automobile headlamps, flood lamps, and photographic lamps. The typical halogen lamp comprises a hermetically sealed light emitting chamber having a tungsten filament mounted internally of the chamber. The chamber is typically formed from a light transmitting material such as glass. The chamber also includes an inert fill gas such as xenon at a pressure of about seven atmospheres at room temperature.
During operation of the halogen lamp, the tungsten filament is heated to incandescence by the passage of electrical current. At full brightness, parts of the filament may reach temperatures as high as about 3000xc2x0 C. while the temperature of the cooler parts of the filament is typically in the range of about 600xc2x0 C. to about 1300xc2x0 C. During lamp operation, tungsten will vaporize from the filament and condense at the cold spot of the lamp which is typically located on the wall of the light emitting chamber. The condensed tungsten causes the chamber wall to progressively blacken during the life of the lamp which undesirably reduces the light output of the lamp during its useful life. It is known to add a halogen to the lamp in an amount to maintain a halogen partial pressure in the lamp sufficient to effect the formation of halogen-tungsten and/or halogen-oxygen-tungsten compounds. The halogen-tungsten and/or halogen-oxygen-tungsten compounds transport the tungsten back to the filament in a cycle known as the tungsten regenerative cycle. Tungsten halogen lamps having a tungsten regenerative cycle have been found to experience reduced wall blackening and thus maintain a relatively constant luminous output during their useful life.
One common halogen used in such lamps to effect the tungsten regenerative cycle is bromine. The known methods for dosing the lamp with a sufficient amount of bromine to effect the tungsten halogen cycle include dosing the lamp with methyl bromide (CH3Br), methylene bromide (CH2Br2), bromine gas (Br2), and hydrogen bromide (HBr). However, the known methods suffer from several disadvantages. For example, methyl bromide and methylene bromide are toxic and also contaminate the lamp with carbon which is detrimental to the performance of the lamp. Thus these compounds are undesirable as a lamp fill material. Bromine and hydrogen bromide are very corrosive and thus undesirable as a lamp fill material. Accordingly, there is a need for a solid lamp fill material including a sufficient amount of a halogen to effect the tungsten regenerative cycle and methods of dosing lamps which obviates the deficiencies of the known lamp fill materials and methods.
The tungsten filament in halogen lamps is susceptible to corrosion due to the presence of the halogen in the lamp. Wells or craters form in cooler regions of the tungsten filament where the filament temperature is in the range of about 600xc2x0 C. to about 1300xc2x0 C. Such well formation may dramatically shorten the useful life of the lamp. It is known that rhenium present at the surface of the tungsten filament inhibits well formation on the filament. Thus it is desirable to deliver a sufficient amount of rhenium to the surface of the filament.
The U.S. Pat. No. 4,413,205 to Ooms discloses that well formation on tungsten filaments may be reduced by forming the filament from a tungsten-rhenium alloy so that small quantities of rhenium are present at the surface of the filament. It is disclosed by Ooms that the well formation on a filament formed from a tungsten-rhenium alloy having one percent rhenium by weight is only very superficial and that no observable well formation occurs on a filament having three percent rhenium by weight.
However, rhenium is an expensive alloying element and lamp filaments formed from a tungsten-rhenium alloy are expensive to manufacture. Further, in an alloy, much of the rhenium is not at the surface of the filament and thus is not effective in preventing well formation. Thus significant cost savings may be realized by delivering sufficient rhenium to the surface of a tungsten filament to reduce well formation rather than forming the filament from a tungsten-rhenium alloy.
The published German Patent Application No. DE 199 29 463 to Dittmer et al. discloses that rhenium present in a lamp from the decomposition of a rhenium halide will deposit on the cooler parts of the tungsten filament during operation of the lamp. The presence of rhenium at the surface of a filament formed from only tungsten has been found to inhibit well formation on the filament The mechanism by which the presence of rhenium inhibits well formation remains unexplained.
According to the present invention, the necessary quantities of rhenium and bromine may be introduced into the lamp in a solid lamp fill material comprising rhenium tribromide (referred to herein as ReBr3 while recognizing that rhenium tribromide exists in the solid and gas phases as the trimeric molecule Re3Br9). Thus rhenium and bromine may be introduced into the lamp without the introduction of any unnecessary compounds or elements which may have deleterious effects on the operation of the lamp.
Rhenium tribromide exists at room temperature as a solid and can be prepared from the elements and formed into a powder. Rhenium tribromide begins to sublime at temperatures greater than about 200xc2x0 C. and begins to decompose into rhenium and bromine at temperatures greater than about 400xc2x0 C. Thus when the rhenium tribromide contacts the cooler parts of the tungsten filament at temperatures between about 600xc2x0 C. and 1300xc2x0 C., the rhenium tribromide will decompose, enabling the rhenium to function as an inhibitor of filament well formation and enabling the bromine to effect the tungsten regenerative cycle.
The amount of rhenium tribromide necessary to deliver the desired amounts of rhenium and bromine into the lamp may vary. Dittmer et al. disclose that a halogen lamp with a volume of 1-2 ml containing a sufficient amount of Re3Br9 or Re3Cl9 to sustain a rhenium halide pressure of 0.3 mbar during operation of the lamp results in the formation of a rhenium layer with a thickness between 20-400 nm on the colder portions of the tungsten filament and leads. Assuming a cold spot temperature of about 700xc2x0 K. (427xc2x0 C.), an amount of 6.7 xcexcg per cubic centimeter of ReBr3 is necessary to attain a pressure of 0.3 mbar ReBr3 in the lamp. Because such small quantities of rhenium tribromide are desired, it is very difficult to dose the small quantities necessary into the lamp with the rhenium tribromide in the form of a powder. Thus there exists a need for a solid lamp fill material in the form of a particle or pellet which may easily deliver the desired quantities of rhenium tribromide into the lamp, and which will release the bromine and rhenium over time.
The U.S. Pat. No. 3,676,534 to Anderson discloses a method of forming particles suitable for dosing lamps with precise amounts of lamp fill material. However, such a method is unsuitable for forming a particle comprising rhenium tribromide because rhenium tribromide will not melt and thus cannot be formed into a particle.
According to the present invention, a pellet may be formed which is suitable for delivering precise amounts of rhenium and a halide in the desired quantities into a lamp by mixing a rhenium halide powder with a powder comprising one or more metals and pressure aggregating the mixture to form a pellet. Thus novel lamp fill materials and methods of dosing lamps have been found which obviate the deficiencies of the known materials and methods.
Accordingly, it is an object of the present invention to provide a novel lamp fill material and method of dosing lamps which obviates the deficiencies of the known materials and methods.
It is another object of the invention to provide a novel lamp fill material for delivering precise amounts of rhenium and a halogen into a lamp without introducing other elements into the lamp.
It is another object of the present invention to provide a novel lamp fill material in the form of a pellet comprising rhenium and a halogen.
It is yet another object of the present invention to provide a novel lamp fill material in the form of a pellet comprising rhenium tribromide and a metal.
It is still another object of the present invention to provide a novel lamp fill in the form of a pellet including only rhenium and bromine.
It is a further object of the present invention to provide a novel method of delivering a precise amount of rhenium and a halogen into a lamp in the form of a pellet.
It is yet a further object of the present invention to provide a novel method of forming pellets comprising a metal and rhenium tribromide.
It is still a further object of the present invention to provide a novel halogen lamp and method of dosing a halogen lamp with a lamp fill material comprising a pellet including rhenium and a halogen.
It is another object of the present invention to provide a novel lamp fill material in the form of a pellet comprising the elements necessary to inhibit tungsten filament well formation and effect a tungsten regenerative cycle in a tungsten halogen lamp.
It is yet another object of the present invention to provide a novel lamp fill material capable of the release of bromine and rhenium over time.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.